A nuclear reactor converts nuclear energy into heat due to radiation of particles and photons. When fissile uranium isotopes absorb neutrons they will split, the fission process, and there is a subsequent release of energy, of fission fragments and of two or more neutrons. These released neutrons can then hit other uranium nuclei causing them to fission and there is a chain reaction going on. The control of the chain reaction is maintained by means of geometric and physical factors such as temperature and presence of moderators and neutron absorbing nuclei. The control rods are a part of this neutron flux control system and are used to regulate the rate of the nuclear chain reaction, by being inserted into or withdrawn out of the core in between the fuel rods or fuel assemblies in the reactor core. The control rods contain material such as cadmium and boron which nuclear structure absorb neutrons.
The neutron flux in the reactor core is carefully monitored and controlled. When the neutron flux decreases control rods are withdrawn out of the core, and fewer neutrons are absorbed and the fission rate increases. When the neutron flux in the core increases, the rods are inserted and the fission rate is decreased. To maintain a controlled nuclear chain reaction, the control rods are manipulated in conjunction with other means in such a way that there is equilibrium between released neutrons and absorbed neutron causing fission reactions.
If the neutron flux in the reactor core is outside predefined conditions the control rods can be inserted in purpose to inhibit the fission process and cause shutdown. This process takes typically only a few seconds. Hence, the control rods constitute a very important part of the nuclear reactor safety system.
If the control rod system or a part thereof for some reason would break down, the reactor or a part thereof becomes uncontrollable. It is therefore of great importance that the control rods and the arrangement to control the rods are inspected with certain time intervals.
The control rods are inserted or withdrawn from the reactor core by some mechanical device which, in its turn, is operated by hydraulic or electric motors etc. All these components are an integrated part of the core assembly and are mounted inside the pressure vessel enclosure of the reactor core and cannot be inspected when the reactor is in operation due to the hazardous radiological environment.
From the U.S. Pat. No. 5,999,583 it is known to analyse the control of the driving mechanism of the control rod by using coil current signals. The analysis is thus directed to a drive mechanism with grippers to engage the drive rod, and leaves analysis of other kinds of driving mechanisms unsolved.
From JP 2004 144604 a method is known to measure a play between a nut and a screw. A worm gear is used to ensure that the nut and screw is self-locking A position sensor is installed in relation to the worm gear and is used to determine changes in torque and the play. Thus, the method requires installation of a special sensor which can be difficult in inaccessible places.
The object of the present invention is to detect faultiness of a control rod and its driving mechanism in a nuclear power plant.